1. Field of the Invention
This invention concerns a pilot valve for controlling a main valve in a main flow line or pressure vessel and in particular to a pilot valve which is non-flowing during normal operation.
2. Description of the Prior Art
A non-flowing pilot valve does not provide direct fluid communication between its inlet or sensing chamber and an outlet fluid chamber, because a pilot xe2x80x9cdome-linexe2x80x9d chamber is either (1) connected to the inlet chamber and the dome-line to the dome in the main valve, (2) is blocked to both, or (3) is connected to the dome of the main valve. The inlet or xe2x80x9csensingxe2x80x9d chamber has a pressure sensing element such as a piston or a diaphragm therein. The main valve, which may be a pressure relief or throttling control valve, normally has a dome with a pressure responsive member therein, such as a diaphragm or piston, which is responsive to fluid pressure from the dome-line chamber of the pilot valve. The dome-line chamber is never in simultaneous fluid communication with both the inlet or sensing chamber and exhaust-line. Thus, in a non-flowing pilot valve, there is no continuous fluid flow through the pilot valve from a bypass line extending from the main flow line, because the inlet or supply line and exhaust-line to the pilot valve are never in simultaneous fluid communication with the pilot dome-line chamber.
U.S. Pat. Nos. 4,682,495 and 4,609,008 illustrate the principles of operation of a non-flowing pilot valve.
The elimination of a continuous fluid flow through the pilot valve removes many of the problems in pilot valves having a continuous fluid flow such as, for example, the clogging of orifices and the like with entrained foreign matter in the flowing fluid, or icing resulting from wet gas applications. While a relatively small amount of fluid may be transmitted intermittently through the non-flowing pilot valve during operation, such amount is insignificant.
Such a non-flowing pilot valve is highly sensitive or responsive to changes in the sensed fluid pressure. The responsiveness of the non-flowing pilot valve to pressure changes in a sensed fluid is relatively constant over its operating range. A non-flowing pilot valve has a fast response to pressure changes in the sensed fluid.
3. Identification of the Object of the Invention
A primary object of the invention is to provide an improved non-flowing pilot valve as compared to prior art non-flowing pilot valves.
Another object of the invention is to provide an improved non-flowing pilot valve which has direct pop seal loading by the set pressure spring to a few percentage points below set pressure so as to provide improved seating through high seat loading.
Another object of the invention is to provide an improved non-flowing pilot valve that has a substantial reduction in part count by providing an assembly of a sense piston and a feedback sleeve, such that reduction in part count results in reduced manufacturing cost, enhanced reliability, simplicity of initial assembly and field service.
The objects of the invention identified above as well other advantages and features of the invention are incorporated in a non-flowing pilot valve having a body with two assemblies. A sense piston assembly is positioned within a cavity of the body and co-axially with the longitudinal axis of the body. A feedback sleeve is positioned co-axially about a neck region of the sense piston. The sense piston and feedback sleeve are free to move axially relative to each other and to the body.
A sense chamber is defined in the body, and a piston head of the sense piston, free to move axially in the sense chamber, is forced downward by a set pressure spring, but moves upward in response to inlet pressure through the piston via passages to the sense chamber. A pop seal carried by the body is disposed axially between an exhaust-line chamber and a dome-line chamber. The feedback sleeve includes a top shoulder, such that below set pressure, the top shoulder engages the pop seal to close a flow path from the dome-line chamber to the exhaust-line chamber. The feedback sleeve also includes a bottom shoulder located at the bottom of the dome-line chamber, and a foot portion of the sense piston carries a reseat seal. The passages in the piston communicate via cross bores to a space beneath the feedback sleeve bottom shoulder and above the reseat seal. Under inlet pressures substantially below set pressure, the space between the bottom shoulder of the feedback sleeve and the reseat seal remains open, such that the dome-line chamber pressure is the same as inlet pressure. As inlet pressure approaches a few percentage points of set pressure, e.g., 98% of set pressure, the sense piston moves up a small axial distance while opposing the force of the set pressure spring, and the reseat seal of the foot of the sense piston contacts the bottom shoulder of the feedback sleeve, thereby xe2x80x9clocking inxe2x80x9d that pressure in the dome-line chamber. A seating spring maintains tightness in both pop and reseat seals. As supply pressure rises further towards set pressure, the sense piston assembly directly lifts the feedback sleeve causing the pop seal to open or separate from the top seat or shoulder. Dome chamber pressure is reduced as its pressurized fluid flows to the exhaust chamber across the pop seat.
Closure is the reverse of opening. As supply pressure decays, lifting force is reduced, the piston moves down and the feedback sleeve moves down thereby closing the top shoulder on the pop seal between the exhaust-line chamber and the dome-line chamber. A short time later, the reseat seal moves below the bottom shoulder of the feedback sleeve, thereby admitting inlet pressure into the dome chamber. This increase in dome pressure, acting on differential areas between the dome chamber and the inlet area, increases lifting force on the piston, and it again moves up, and the reseat seal of the piston again seats on the bottom shoulder of the feedback sleeve again xe2x80x9clocking inxe2x80x9d dome pressure.
The non-flowing pilot valve of the invention is designed with effective cross section areas or diameters A1 (of the sense chamber), A2 (of the pop seal or of the reseat seal), and A3 (of the inlet area) such that a feedback ratio, F=A1/A2xe2x88x92A3, is a characteristic of the level of sensitivity for the spring of the pop seal and the closing of the reseat seal. A feedback ratio in the range of 5 to 10 for the non-flowing pilot valve and the provision of a seating spring between the piston and the feedback sleeve are features which reduce simmer or leakage of the valve before opening and closing.